A few days afterwards, he learnt that the latter were concentrating on a single point their troops and the contingents of the tribes under their dependence. He provided for the supply of provisions, chose a favourable position for his camp, and enjoined the Ubii to transport their cattle and goods into their _oppida_, hoping to compel the barbarians by famine to fight at disadvantage. The Ubii were similarly charged to watch the enemy by means of numerous scouts. A few days later, they informed Cæsar that the Suevi, at the approach of the Romans, had retired, with all their troops and those of their allies, to the extremity of their territory. There lay the forest Bacenis,[452] which advanced very far into the country, and which, placed like a natural barrier between the Suevi and the Cherusci, separated these two peoples and defended them against their mutual excursions. It was at the entrance to this forest, probably towards the mountains of Thuringia, that the Suevi had resolved to await the Romans. In this expedition, as in the one preceding, Cæsar feared to engage himself too far in the middle of an uncultivated country, where provisions might have failed him. He therefore repassed the Rhine. But to keep the barbarians in fear of his return, and to prevent their re-enforcements from reaching the Gauls, he did not destroy the whole bridge, but only cut off 200 feet on the side of the Ubian bank; at the extremity of the truncated part he built a tower of four stories, and left on the left bank twelve cohorts in a retrenched post. Young C. Volcatius Tullus had the command of it. Cæsar’s two expeditions to the right bank of the Rhine led to no battle, and yet the moral effect was so great, that after this period the Germans no longer supported the insurrections in Gaul, and even became the auxiliaries of the Romans.[453] [Sidenote: War against Ambiorix.] VI. On the approach of harvest, Cæsar marched against Ambiorix, with his ten legions, except the guard left at the bridge of the Rhine. He started from Bonn, and advanced towards the country of the Eburones, by way of Zulpich and Eupen (_see Plate 14_), across the forest of the Ardennes, which extended, it will be remembered, from the banks of the Rhine to the country of the Nervii. In the hope of surprising the enemy, he sent forward M. Minucius Basilus, with all the cavalry, recommending to him not to light fires, which would reveal his approach, and informing him that he should follow him closely.